This invention relates to a method and apparatus for determining the equivalent blackbody color temperature of incandescent lamps. In many applications of incandescent lamps, possession of measured values of filament color temperature would be a valuable design aid through reducing time investment and improving accuracy in the meeting of design requirements.
For example, to design a lighting system in which lamp output is to be "whitened" by blue filtration to meet a given chromaticity specification requires that filament color temperatures be accurately known, unless a cut-and-try approach is used. This is true because relatively small variations in filament color temperature result in widely divergent resultant chromaticity of the filtered light, and because there are no consistent correlations between color temperature and other lamp parameters, such as average luminous intensity or filament power.
An accurate straight-forward lighting system design approach would consist of the following steps: (1) measure the color temperatures of a statistically significant number of lamps of the selected type to obtain a distribution curve and a color temperature norm; (2) design a filter to achieve the desired chromaticity coordinates with maximum lamp utilization; and (3) select lamps for use in production by measurement of color temperature and application of tolerance limits based on filter and lamp characteristics and the margin of error allowed for the chromaticity of the filtered light. The apparatus and method of the present invention may be used in carrying out steps (1) and (3).
To accurately specify the chromaticity of an unknown light source, the relative power levels corresponding to each of three standard Commission Internationale de l'Eclairage (CIE) primary color components (red, green and blue) must be determined by some means of spectral analysis.
If a source has a known distribution of radiant exitance relative to wavelength, it may be possible to simplify the color specification to a single-valued parameter.
This is commonly done with blackbody radiators, because the radiant exitance as a function of the absolute (Kelvin) temperature of the radiator is known through Planck's equation, which is EQU M.lambda.=C.sub.1.sup..lambda.-5 (e.sup.C.sbsp.2.sup./.lambda.T -1).sup.-1,
Where: M.lambda. is the total spectral radiant exitance in watts per square centimeter of emitting surface area per micron bandwidth at wavelength .lambda. in microns (dimensionally, energy per unit time per unit length cubed); .lambda. is the wavelength in microns; T is the absolute temperature of the blackbody radiator in Kelvins, e is the base of the Napierian system of logarithms (2.7182--); C.sub.1 is a constant (3.7418--.times.10.sup.-12 watt-cm.sup.2); and C.sub.2 is a constant (1.4388--.times.10.sup.4 micron-Kelvins).
Any given blackbody temperature of sufficient magnitude to generate visible light would correlate to a particular unique set of tristimulus color coordinates. The absolute temperature of a blackbody radiator, then, may be used in lieu of the corresponding tristimulus color coordinates to specify the chromaticity of its radiated light.
The overall spectral distribution of power radiated from an incandescent lamp differs considerably from that of a true blackbody. For this reason, the lamp is called a "selective radiator."
In the visible part of the spectrum, however, the incandescent lamp power distribution curve almost tracks that of the blackbody, but at a lower per-unit-area radiated power level. If only visible radiation is of concern, the lamp comes very close to qualifying as a "graybody," because Planck's equation may be applied through use of an appropriate efficiency factor which is nearly constant as a function of wavelength throughout the visible spectrum.
At a particular filament temperature, the shape of the spectral power distribution curve of the lamp will be practically identical to that of a blackbody operating at some higher temperature. Regardless of power levels, a match of distribution shapes implies a match in chromaticity.
The color temperature of an incandescent lamp is the absolute temperature of a true blackbody that causes the blackbody to generate a spectral distribution of radiant power that is the closest possible match to that of the incandescent lamp.
Color temperature is universally used as a means of specifying the chromaticity of incandescent lamps and is expressed as equivalent absolute temperature in Kelvins.
A comparison of the standard (CIE) tristimulus chromaticity values for a black body radiator over the range of 1600 K. to 3200 K. (the practical range for incandescent lamps) shows that relative to red, the green component changes very little whereas the blue component undergoes a drastic change, steeply increasing as the temperature rises. Table 1 is a tabulation of the tristimulus values, based on a chromaticity analysis of Plank's equation for several values of absolute temperature, wherein x, y and z represent, respectively, the red, green and blue components of chromaticity values expressed as normalized or fractional parts of the total radiated power at each tabulated temperature (sum of x, y, and z equals unity to three decimal places for each temperature).
TABLE 1 ______________________________________ Spectral Analysis of Blackbody Radiator CIE Chromaticity Functions Applied To Plank's equation T (K) x (Red) y (Green) z (Blue) Ratio z/x ______________________________________ 1600 .573 .399 .027 .047 1800 .549 .408 .042 .077 2000 .527 .413 .060 .114 2200 .506 .415 .079 .156 2400 .486 .415 .099 .204 2600 .468 .412 .119 .254 2800 .452 .409 .139 .308 3000 .437 .404 .159 .364 3200 .423 .399 .178 .421 ______________________________________
Table 1 includes a ratio column which shows that each temperature value has a corresponding unique ratio of blue-to-red components which is independent of the level of the total power radiated by the source.
FIG. 1 is a plot of the blue-to-red ratio as a function of temperature, using values taken from Table 1.
The functional relationship between the blue-to-red ratio and color temperature can be used to form the operational basis for a simple bi-color instrument to be used for measurement of the color temperature of incandescent lamps.
The blue and red filter bands need not be those as defined for the CIE tristimulus specification of color. The bands may be selected to obtain the best compromise among such characteristics as ratio curve slope and linearity and signal-to-noise ratio.
To form a complete instrumentation system, the computed blue-red ratio signal may be coupled into a calibration "translator" circuit capable of linearizing the input and setting slope and offset values so as to drive a digital display to deliver direct read-out of the color temperature in Kelvins.
Such a system can be calibrated through use of lamp standards that may be obtained from the National Bureau of Standards (NBS). These lamps have color temperature specified as a function of filament current for several points. In addition, and conversely, NBS will provide a polynomial equation having computer-derived coefficients which allows computation of filament current for any specified color temperature over a wide range of practical values.
It is an object of the present invention to provide a high-reliability, all solid-state electronic measurement system, wherein motor-driven filter wheels and servo-controlled shutters are not used.
It is another object of the invention to provide a lamp analyzer requiring infrequent calibration, which is uneffected by ambient temperature changes typical of a laboratory environment, and which is completely automated, eliminating the need for manual gain adjustment, nulling procedures or meter zeroing.
It is a further object of this invention to provide a lamp analyzer which has a minimum luminous intensity dynamic range of 2000 to 1, with sufficient sensitivity to allow accurate color temperature measurement at average luminous intensities of about 10 mcd (millicandelas) or less and having a color temperature range of 1600 to 3200 Kelvins.
It is yet another object of this invention to provide a lamp analyzer having an error detector providing for display shut-down to prevent erroneous temperature read-out if the luminous intensity or color temperature of the lamp is below the threshold of accurate signal processing.
It is a further object of this invention to provide a lamp analyzer having a simple and straightforward calibration procedure with all adjustments being independent and non-iterative.
It is still another object of this invention to provide a lamp analyzer having a direct color temperature read-out in Kelvins on a digital display having a resolution of four digits, wherein the color temperature error is small compared to the estimated tolerance of the NBS calibration standard over the full range of the analyzer.
It is a further object of this invention to provide a lamp analyzer wherein the light radiated from the lamp is integrated before subjection to ratio analysis in order to average the effects of filament temperature gradient.
It is yet another object of this invention to provide a lamp analyzer with blue and red channel electrical inputs which are derived from the same or identical radiated light samples.
It is still another object of this invention to provide a lamp analyzer wherein the blue and red channels have virtual linearity over the full dynamic range of the instrument, and which have electrical gain values that are independent of ambient temperature or that precisely track one another with changes in ambient temperature.
It is a further object of this invention to provide a lamp analyzer with a gain control system wherein the blue and red channels precisely track with one another over the range and sensitivity of the lamp.
Other objects, advantages, features and results will more fully appear in the course of the following description.